1. Field of the Invention
The invention relates to a method of through-switching broadband digital signals without phase jump, in a synchronous broadband communication network which includes asynchronously operated space-division-multiplex switching networks, in which the broadband digital signals are regenerated individually.
2. Prior Art
In future integrated communication networks, new services are planned which require larger bandwidths than the narrowband ISDN-services (Integrated Services Digital Networks). Picture telephones (video telephones) require a bandwidth of 5 MHz or a transmission rate of a few Mbit/s to some 10 Mbit/s per channel. For reliable operation of such broadband networks, synchronization measures plan an important part. In node-synchronization, the network clock which is the central clock for through-switching in the relevant node is used to provide synchronism between incoming signals. In source-sink synchronization, the frequency difference between the source reference clock and the central network clock is transmitted, for example as a delta coded auxiliary signal in the multiplex signal, to the sink to enable a clock synchronization of the sink. The clock synchronization is independent of the network clock and which uses a stuffing method.
To synchronize a circuit, the source clock is transparently conveyed through the network to the sink. To synchronize a network, the clock for the sources is supplied from the network. In broadband communication networks, the first node, close to the subscriber, is often still used in common for broadband and narrowband transmission. The node located at the terminating point of, for example, the optical fibre transmission path comprises a multiplexer/demultiplexer through which there is access to the broadband switching network, to the voice/data (ISDN) network, and to the program coupler in the distribution networks. The broadband switching network, the voice/data (ISDN) coupling arrangements, and the program coupler are controlled by a control unit included in the node. Separate transmission of the respective digital signals in the broadband and the narrowband networks is effected from this common node. For switch-through in the broadband switching network, a space-division-multiplex switching network is often used. A time-division-multiplex switching network is often used for the voice/data (ISDN) coupling arrangement. To synchronize this node, the central network clock is applied to it via the standard frequency distribution network. In the periodical "net", Vol. 39 (1985), No. 1, pages 20 to 25, the system architecture for a broadband communication network and the structure of a broadband coupling arrangement (broadband switching network) is described in detail. As can be seen therefrom, the time-division-multiplex method used for switching purposes in narrowband switching networks (voice/data (ISDN) switching network) will not be advantageous in the near future for broadband signals up to 140 Mbit/s, as the cost and design efforts for frame and clock phase synchronization will be excessive. Thus, broadband switching networks are presently implemented in the space-division-multiplex mode. Page 24 of said publication discloses the structure of an asynchronously operated space-division-multiplex switching network.
In accordance with the present state of the art (cf. for example, "Elektrisches Nachrichtenwesen", Vol. 58, No. 4, 1984, pages 450 to 452) only space-division-multiplex switching networks are suitable for use as broadband switching networks. While time-division-multiplex switching networks require a synchronous-mode operation using the central node clock (office clock), it is in principle possible to use either synchronous-mode operation or asynchronous-mode operation in space-division-multiplex switching networks. Synchronous-mode operation is the most obvious mode for a clock synchronous broadband network, in which the node clocks can be derived from the already existing standard frequency distribution network. Synchronous-mode operation with common clocking of the signals during traversing of the switching network is, however, not without problems as, for high bit rates in the spatially extended (multi-stage) switching network, supplying the central network clock in its proper phase requires complicated adjusting measures. In the above-mentioned publication in "net", Vol. 39 (1985), No. 1, page 24 it is mentioned in this regard that because of the spatial expansion of the switching network arrangement and the high bit rate of the broadband signals it is not possible to regenerate the signal by a central clock at any desired position at low cost. Consequently the space-division-multiplex switching network is operated in the asychronous mode with individual clock processing for regenerating the signals during traversing of the switching network by means of a "clock-preserving" line code. A code of the type 5B-6B can be used as "clock-preserving" line code. The CMI (Coded Mark Inversion) interface code recommended by the CCITT can alternatively be used as the transmission code. The asynchronous-mode operation of the space-division-multiplex switching network has the advantage that it has a larger tolerance as regards time-delay jitter of the switching network components, but has the disadvantage that after through-switching of a circuit all the subsequent clock regenerators of the same hierarchic stage must synchronise themselves with the new clock. If a plurality of digital lines are to be combined in one digital multiplexer, then asynchronous multiplexers operating in accordance with a stuffing method are used. (cf. "ntz", Vol. 38, (1985), No. 3, pages 140 to 144).
In time-division-multiplex exchanges, the incoming data signals coming from a synchronous data network are taken over by the time-division-multiplex exchange with a clock determined by the modulation rate of these signals. After switch-through of the data signals in the time-division-multiplex exchange, these signals are conveyed with a correspondingly higher clock. If clock signals from different clock sources, which are plesiochronous with respect to the clock source of the time-division-multiplex exchange, coincide at the interface between the time-division-multiplex exchange and the data lines, then a clock adaption must be effected. The published German Patent Application DE-AS No. 24 20 437 discloses a method of adapting the clock of such plesiochronous signals. In that method, the incoming data signals are brought into phase with the clock of the time-division-multiplex exchange by means of line circuits arranged at the input end of the exchange. At the output end of the time-division-multiplex exchange, the data to be transmitted are again brought into phase with the clock raster of the time-division-multiplex exchange by means of a clock which is offset by the basic transit time of these data through the exchange. This clock has the same frequency as the clock at the input end. Afterwards the data are conveyed further.